In March 2011, a killer earthquake shook the seafloor off the eastern coast of Japan. It triggered powerful tsunami waves. Some towered more than 40 meters (131 feet) by the time they hit the coast. Where the land was especially flat, those waves roared more than 10 kilometers (6 miles) into Japan, killing thousands of people. But Asian waters weren’t the only ones that got sloshed around.

Right after the magnitude-9 quake, scientists knew that its tremors had set distant waters in northern E...